hen not acting right

I got 3 chicks in march they are golden comets 2 of them started laying a few months ago the one that didn't has always ate well and is the same size as the other 2 but her comb never filled out and is just pink not big and red like her sisters I added 5 more to those 3 about a week ago that are 4 months old she has recently started to lose the feathers from the back of her neck I wormed all of them about a week ago yesterday she just started standing off from the others by herself she still seems to be eating and drinking and there was really no problems with adding the new hens just not sure why she hasn't started laying like her sisters